1. Field of the Invention
The present invention relates to the field of vehicle navigation along a predetermined pathway.
2. Prior Art
In certain applications it is desirable to provide a driverless vehicle which will follow one or more predetermined paths by itself. One known approach for achieving this objective is to provide a conductor along the path the driverless vehicle is to follow and to sense on the vehicle the magnetic field caused by a varying current in the conductor to locate the vehicle at any time with respect thereto. Such a system is described in U.S. Pat. No. 3,039,554, with the second paragraph of the patent providing a good description of the basic concept of such systems. As generally stated therein, the route defining conductor may be secured on or laid just beneath the surface of the ground, floor, roadway, etc. Normally the electrical signal for steering the vehicle is an oscillating or AC current which produces a corresponding alternating magnetic field in the vicinity of the route defining conductor. Two sensing coils on a vehicle proceeding along the route defined by the conductor are so arranged that the magnetic field which exists in the vicinity of the conductor induces signals in the sensing coils, which signals are used to control the steering mechanism of the vehicle. The arrangement is such that when one of the signals derived from one of the coils is larger than that derived from the other, the steering mechanism operates to redirect the vehicle toward the route defining conductor. The resulting movement of the vehicle causes the positions of the sensing coils to change in relation to the conductor, and thus the steering mechanism is controlled in such a way as to tend to maintain equality between the voltages induced in the two sensing coils. Systems of this general kind, with variations and/or embellishments, are disclosed in U.S. Pat. Nos. 4,010,409, 4,310,789, 4,215,759, 4,656,406 and 4,736,812.
In the foregoing systems, the current in the route defining conductor is generally provided by some stationary power source, though included in the foregoing patents are means for switching for defining branches in the pathways. In U.S. Pat. No. 4,855,656, the current in the route defining conductor is not provided by a stationary source, but rather is induced therein from the vehicle itself. Thus in this patent, means are provided on the vehicle for inducing a current in the conductor, with a second means on the vehicle separated from the first (again using two sensing coils) sensing the magnetic field caused by the current induced in the conductor.
U.S. Pat. No. 4,284,941 discloses a system for data communications between fixed and moving terminals wherein a conductor connected to the fixed terminal and disposed in the travel path of the moving terminals supports lateral guidance control information for the moving terminals as well as two way inductive data communications between the fixed terminal and the moving terminals. The fixed terminal transmits over the conductor an alternating current lateral guidance signal modulated in accordance with longitudinal control information for each of the moving terminals. Each of the moving terminals, in turn, transmits status information over the conductor to be used for developing control information at the fixed terminal. The fixed terminal employs hybrid transformer techniques in conjunction with precision filtering to enable reception of low level status information from the moving terminals and concurrent rejection of the modulated lateral guidance signal and other interfering signals.
In the foregoing systems using a single path defining fixed conductor and a pair of sensing coils on the moving vehicle, one normally servos the steering to the output of the sensing coils to drive the difference in output of the two sensing coils to zero. In certain applications however, it may be desired to provide some form of additional steering control on the vehicle. By way of example, in U.S. Pat. No. 4,284,941 herein before referred to, a controller on the vehicle may be used to effect travel path selection. In other situations it may be desired to control the path of the vehicle relative to the conductor, at least within reasonable limits. By way of example, the preferred embodiment of the present invention is used in conjunction with a toy road raceway set wherein each of two players may control a respective one of two battery operated race cars on a flat (unslotted) race track. Thus, in the present invention it is desired to have each player be able to steer the respective race car down any part of the full width of the track, so that the racers can jockey for position in the turns, pass each other on the straightaways, etc. While some control over track position could be achieved in the prior art systems by servoing the difference in the sensor coil outputs to a controllable offset, the extent of control which may be achieved in this manner is limited, primarily because of constraints in the geometry of the system. In particular, so long as the axes of the two sensing coils on the vehicle stay clearly on opposite sides of the conductor, reasonable control can be obtained in this matter. However, this does not allow sufficient maneuvering for passing and for meaningful control of the vehicle going into and coming out of turns. For greater excursions however, gross nonlinearities and even instabilities may develop because of the declining signal in the outer sensing coil, and a polarity reversal in the inner sensing coil as its axis moves over the conductor so that the axis of both sensing coils are on the same side thereof. Thus, these single wire systems of the prior art have been found inappropriate for lateral control to the extent desired for applications such as, by way of example, a toy race car set.